Contact the parents by phone if possible. In a calm manner, state the facts without placing blame. Set a time when you can meet to discuss the bullying problem. Never show up at a person's home and demand their attention at that moment.
Meet in a public place or even at the school. At the meeting, speak in a calm, yet assertive manner on what you and your child are experiencing. Keep your emotions in check, and provide actual instances of when your child was bullied by the other child. Take into consideration that the other parent may not be aware of what his child is doing.
Listen to the parent's side of the story, and be willing to hear things your child may have done. Avoid becoming defensive. Let the other parents know that your main goal is to find a way to resolve the conflict going on between the children. Take time to brainstorm how everyone can change the situation.
End the meeting if the parent becomes confrontational or argumentative or is unwilling to remain calm and discuss resolution. Simply say that you would like to end the meeting for now and try again with a mediator.
Agree to a second meeting, once you've agreed to a resolution. The second meeting will provide a chance for follow up, adjustments to the resolution or a check-in to see how everyone is coping with this issue. If the bullying has stopped, no further meetings are needed.